This invention relates to a method and apparatus for the formation by extrusion of a laminate pipe having respective layers of metallic and plastics materials, especially a triple walled pipe having inner and outer layers of plastics material and incorporating between them a metallic layer. Such multilayered pipes are in themselves well known, as are methods and apparatus for their production by extrusion. Typical examples are given in EP-A-0024220, EP-A-0237234, U.S. Pat. No. 3,376,181, WO-A-88/03084, FR-A-2434326, FR-A-1508630, GB-A-2223427 and U.S. Pat. No. 4,370,186.
However, in these prior art constructions, the plastics material is adhered to the metallic layer either by incorporation of an interlayer of an adhesive between each of the layers of plastics material and the metal foil (see, for example, GB-A-2223427) and/or pretreating the metal foil with a strong acid or alkali (see, for example EP-A-0237234) to provide an etched metallic substrate.
Such constructions are said to be advantageous during the final working up stages during production of the multilayered pipe, which is capable of preventing diffusion of oxygen radially through it.
Typical of the prior art is the extrusion assembly disclosed in U.S. Pat. No. 4,370,186. In this apparatus, a U-section form metallic strip is led around a first extrusion apparatus for extruding respective inner layers of adhesive and plastics material and formed, by means of a shaping device, into a metal tube surrounding a nozzle of the first extrusion apparatus, the tube being welded upstream of the end of the nozzle running through the tube. Respective concentric layers of molten plastics material forming an innermost plastics layer and an adhesive layer respectively are extruded through an annular section gap between an outer wall of the tubular nozzle and a centrally located die and out of the nozzle through an exit orifice. As the sealed tube passes the exit orifice it is coated by the adhesive and plastics layers to provide respective inner layers. The die has a centrally disposed passage through which pressurized air can pass. This serves to force the layers of plastics material against the inner surface of the tube at a position downstream of the tubular nozzle. As shown with particular reference to FIG. 3 of U.S. Pat. No. 4,370,186, a second extrusion apparatus downstream of the first extrusion apparatus providing the inner layers, provides respective concentrically disposed outer layers of adhesive and plastics material. A pipe such as that constructed by the method using the extrusion apparatus of U.S. Pat. No. 4,370,186 is now described with reference to FIG. 1 herein, which shows a pipe having a series of concentrically disposed layers A-E, A being an innermost bore layer of plastics material, B a graft adhesive polymer layer, C the metallic layer, D a second graft adhesive polymer layer and E an outer sheath layer of plastics material. The metallic layer C may be of aluminium or other metal, typically metallic foil. In variations of this process, either instead of or in addition to the provision of the adhesive layers B and D, the metallic layer may be etched; see, for example, EP-A-0237234.
Although commercially acceptable piping may be produced by such methods, there are problems associated with using an adhesive, for example, grafted polyethylene, or etching treatment, in securing an overall even thickness.
Moreover, where grafted polyethylene is utilized, a typical thickness would be 0.10-0.15 mm on each side of the metal foil, adding a total sectional thickness of 0.2-0.3 mm to the outer diameter of the pipe.
In addition, during the co-extrusion coating of thin adhesive grafts onto metal foils, problems occur where the adhesive fails to cover the entire area with an even and identical thickness. Thus, when the next layer of plastics material, for example polyethylene, is applied over this graft, bridging of the polymer between adjacent areas of the graft can occur, or zero bonding may occur where no graft exists. This can be seen, for example when a clear sheath is applied over a coloured graft material. This leads to premature hoop failure under hydrostatic pressure. This ability to withstand hydrostatic pressure is known as the hoop strength. Inefficient bonding of the plastics material to the metallic layer by the adhesive can reduce this hoop strength by as much as 20% at ambient temperature, for example from 100 to 80 bar; the ductility of the metal foil will always fail in the area of least graft material.
Another problem associated with the use of grafted polyethylenes as adhesive materials is their relatively low Vicar temperature, measured according to BS2782/102/D/F/J, which for such polyethylenes is in the order of 50.degree. to 85.degree. C. Thus although a crosslinked polyethylene which may form a bore and/or sheath wall of a triple-walled pipe may have a Vicar temperature as high as 120.degree.-140.degree. C., allowing excellent thermal performance whereby the pipe could be used at temperatures within a wide range, such thermal performance is severely limited by the low Vicat temperature of the adhesive which causes a loss of adhesion at high temperature.
Where attempts are made to achieve efficient bonding by etching treatment of the metallic layer, although the tolerance varies less, craters are caused by the etching, which leaves unfilled voids bridged by the polymer. Moreover, although etching treatment produces an improved temperature resistant bond, it is nevertheless environmentally unfriendly and can be neutralised with the piped medium through permeation, again causing hoop stress decay. A yet further disadvantage is that once the foil is treated the etching is prone to handling problems associated with deactivation of the surface.
It can be seen from the above that if it is desired to produce a triple walled pipe which is dimensionally accurate, mechanically and thermally stable in relation to the prevention of delamination and which can perform at considerably high temperatures and pressures, then none of the above processes could be said to be suitable for obtaining optimum mechanical and thermal properties with high chemical and frictional resistance. For example, in the construction of an "ideal" pipe, its optimum physical strengths are attainable only if the overall cross-section of the wall thickness is identical throughout the pipe length, while keeping the wall section thickness to a minimum. Thus, where the wall thickness tolerance varies by more than plus or minus 0.5%, the physical properties of the pipe will be decreased by as much as 20% or more.